ECM schematic or servicing info?

You won't be finding a schematic for the ECU, anytime soon. They are very rare and just about never get out of the OEM's research labs.

In 1990 the ECU was somewhat simple minded. The diagnostic error code output was nothing more than a series of pulses, on one pin of the diagnostic connector. The intention was that any mechanic with an analogue voltmeter could read the diagnostic codes. After 1995 the diag codes became more complex and usually required a diagnostic code reader (code scanner)(not on your car, but cars manufactured after 1995!!) The ECM learns your driving habits and the characteristics of the car in about a week, then retains this info until your reset the ECM memory. That's why your car runs poorly for the first week. I'll bet that, based on what you have said here, there is nothing wrong with the ECM. The check engine light is indeed pointing to another fault somewhere in the car. I don't have the specific pin out for your ECM, but I'll GUESS that the diag connector is located next to the fuse block on the left kickpanel (on left hand drive vehicles) under a black protective rubber cover. All you'll need to check for error codes is a volt meter (12 volts) and the correct pin location. Check the service manual (see your local library for the manual). The manual will have the error codes, their description, contact points, and procedure for reading the codes.

Some ECM's of this vintage had a problem with the capacitors leaking, and eating up traces on the PCB. While your into the repairs, you may want to at least inspect the caps for leakage, swelling, or other damage. Typical failure, results in the vehicle being totally dead and not running (cranks OK, just won't run). Replacement caps are typically just a few dollars, and easy to replace.

For further reading, you may want to try this site. It's for Eclipse and Talon vehicles, but it will have information on Mitsu ECM cap repairs.

Good luck

WOW, awesome info on that website! And, I thank you very much for the info you gave.

I have the service manual (Chilton, for 1990 Colt), and it describes the ECU inputs and pin out on the connector. It also describes how to read the error code with a simple DVM or analog voltmeter.

Here's the problem.....

A local dealership told me they couldn't work on the check engine light problem without being able to communicate with the ECU. They told me my ECU isn't talking to the tester they have although it runs the car just fine. I was put under a lot of pressure to buy a new ECU, so they could start to work on the check engine light problem. The mechanic himself told me he should be able to connect his tester to the ECU and interrogate it to get various data from it in order to diagnose the problem.

The example he gave was the throttle position sensor. He said he should be able to read the throttle position from the output of the ECM. IF it appears to be stuck when it's actually moving, then he has to determine whether the sensor is bad or whether the ECM has an electrical problem.

He also said he had to know the setting of the motor that maintains the idle speed...which he said the ECM would report if it was asked for this data by the test instrument.

So, I was told they could not diagnose the check engine problem without proper communication with the test hardware....which my ECM was not providing.

After I read the service manual, I learned about manually checking the stored error codes and I did the procedure myself, bought a new oxygen sensor and installed it myself. The problem went away for 3 or 4 months, but then the check engine light started coming on intermittently (again).

So, I'm back where I started....at least I didn't spend big bux for an ECM that I probably didn't need.

I had a serious argument with the Service Manager about the ECM functionality. I told him to read the error codes manually rather than relying on the ECM's data communication with his test instrument. He told me they didn't use second rate diagnostic procedures and gave me the car back after declining to work on it further.

So......I'm a little confused!! Can the dealer be that fubar'd, or is there indeed a diagnostic data out pin that they use to interrogate the ECM with??

Again, it's a 1990 Dodge Colt, 1.5 L engine with 216K miles on it.

Thanks for your help.

Pre 1995 ECU's (OBD1) did not "generally" have (or need) input diagnostic ports. All they needed to do was output an error code. HOWEVER, there are third party "Data Loggers" that will work with Colts and allow for some control of solinoids, injectors etc.

not sure what this mechanic is thinking of,(I wouldn't want to think he'strying to make an extra buck here). A good mechanic wouldn't depend onhaving a special scan tool for an OBD1 car.Even the Mitsu., shop manual for my son's 1991 Laser describes using ananalogue meter to read the codes. Nothing is mentioned at all about beingable to "Interrogate" the ECM with a special tool (as is needed for my 2003Eclipse). I would suggest you read the error codes again, and maybe post the codes here, and see what the problem is. With the mileage you have, I would check all sensor connectors for mositure or corrosion. Even just unplugging then replugging them in may clear the problem.

The idle position sensor is nothing more than a rheostat (potentiometer), and very easy to test with an ohm meter.

Most mechanics should know, that when you unplug the ECM from the battery you will lose the memory and the car (ECM) will need to relearn your driving habits. This is common across many different lines of auto's, and usually takes about a week, depending on your driving habits. You may well have a different problem here than you had last time. In almost all cases you can test the electronic sensor items in your colt with a simple VOM, instructions will be in your Chiltons manual. It is my opinion that there is nothing wrong with your ECM (Capacitor issue aside). You may want to change dealerships though, I think I'd be a little pissed with your current dealer also. They'd look pretty bad if they fixed your car now, after telling you that you needed a new ECM, when you really didn't.

OK, many thanks Niro!

I don't know for sure that the dealership was using a Mopar test instrument, or whether it was an aftermarket brand. But I do know I was treated like I had some kinda disease when I suggested reading the error codes manually and the treatment was even more hostile after I showed the Service Manager the procedure in the Service Manual.

He very much wanted me to fix the ECM before proceeding, and as I said before, I was invited to take my car elsewhere when I suggested using the low tech method for reading the error codes.

I am a little worried though...about the ECM. After reading about the electrolytic caps, I'm wondering if I am on the verge of a catastrophic ECM failure.

I note that the check engine light sometimes goes off and stays off.....which (I think) means the ECM has reset itself. I also notice occasional skipping, which I had thought to be worn out ignition wires since it happens alot in wet weather (but occasionally when it's dry).

A couple times in the last week, the ignition cut out completely for a quarter second or so while driving. It came right back on and didn't sputter after that. During the brief time the ignition winked out, I saw the tach drop out completely, even though the motor was in gear and still turning over... So, I'm wondering if the ECM has bad electrolytic caps and a catastrophic failure is about to happen.

I should say that the check engine light did not come on following the brief drop out of the ignition....perhaps this is significant, perhaps it isn't.

It sounds like I need to order some capacitors and do a preemptive capacitor lobotomy on the ECM (just in case).

I did see the pictures and read the web material describing how to change the caps and what type of caps to order. Fortunately, I am fluent in soldering and have electronics training...so it's a minor operation for me.

Thanks for all the suggestion and guidance. It's much appreciated.

M

When your changing the caps, check the circuit traces under the caps for damage, such as cracks or breaks in the copper. Some of the caps I've changed had a glue like material around the base, which could damage the trace on removal.

Chrysler Canada quoted me $2000.00 cdn, for a new ECM for a 91 Laser, and at that it had to be special ordered from the center of the universe, I can see why they wanted you to replace it.

If the ECM loses the pulse train from the TDC and crank sensor, it will not fire the ignition coil. This would kill the tach, and the engine would stop. When the signals return the ECM would continue to fire the ignition coil and the engine would continue to run. This doesn't normally light the maintence light. Have you checked the connectors? These signals are low voltage/current, and it doesn't take much to affect them. While you may have a problem with the caps, I believe you have a problem elsewhere. You have something that is borderline, as the ECM clears the trouble light when the "sensor" falls back into spec. Maybe an O2 sensor or airflow sensor, best to check the error codes. If you can't get the error codes, use the shotgun method and test all the sensors in the car, there really isn't that many. Your Chiltons manual will tell you how to test them. And don't rule out that O2 sensor you replaced some months ago until you've tested it.

Remember that you can use larger capacity (microfarad and voltage) caps, than whats there now. The caps are all used on the power rails of the ECM. I do recommend that you replace the caps on spec, but if they don't look bad you could probably get away with keeping them.

Hey Niro,

You sound like a pro.......do you work on these professionally??

I have some training in electronics, and find myself wondering WHY they use multiple electrolytic caps on the 12 volt rail inside the ECU box. These days, the whole system should be powered by 5 volts or even

3.3 v. Certainly the microprocessor and the eeprom should be 5 or 3.3 volts. The ECM should source all the power needed by the sensors as well, so the sensors only need to be grounded externally to work properly. Thus 12 volt electrolytic caps should not be needed at all, especially within (inside of) the ECM enclosure.

I know the automotive power system leaves a lot to be desired and it's a hostile environment for all electronics.....after all, an alternator puts out pulsating DC which is smoothed by the internal capacitance of the battery and whatever capacitors are on it's output. But, using electrolytic caps for this smoothing is a bad idea.

It's bad because the input voltage is always pulsing, so ripple current is high within the caps. This factor alone could be responsible for the electrolytic caps failing....because the ripple is relatively high frequency and because it never stops, the capacitors have to work very hard to smooth out the pulsating DC input.

A much better way is to feed the 12 volts into a regulator, which drops the voltage down to 3.3 or 5 volts. A switching regulator could be used, but is probably not necessary because the power drawn by the ECM is actually quite low. The regulator would do the job of smoothing out the voltage supplied to the ECM electronics without involving large (high value) electrolytic caps.

If a simple 7805 voltage regulator was used, it would need .1 or .2 uF caps on the input and the output. These caps could be disk ceramic, which are practically indestructible! Since the regulator electronics does the bulk of the 'filtering', any capacitors on the 5 volt power rail would never see large ripple currents, and they to would last nearly forever....and generic standard electrolytic(s) could be used. My guess is that the high temp electrolytic(s) are only necessary due to the large ripple current the caps see (as a result of the relatively crude alternator output).

I'd be willing to bet the temperature inside the ECU is never very high, and that the high temp electrolytic(s) are only needed because of the high ripple current they see.

Even a simple high value inductor on the input of the ECU power feed would reduce the ripple current seen by the caps. Although one could argue that a regulator requires a complete redesign, a 1 H choke in series with the 12 V power line could easily be placed inside the ECM during reconditioning/repair. The choke would reduce the ripple current seen by any electrolytic(s) placed on the 12v rail inside the ECM. And, the choke would not wear out! The presence of the choke would improve the lifetime of the electrolytic(s) by minimizing the ripple current they see.

By utilizing the magnetic field stored by the inductor, the inductor could (potentially) totally eliminate the need for electrolytic(s) to do the filtering. When the input voltage drops, the extra energy needed to keep the output voltage at 12v could be drawn from the stored energy in the inductors magnetic field. Using an inductor to smooth out the voltage fluctuations would be a great improvement in reliability.....an inductor should last forever (literally).

If I was working on this problem professionally, I'd put an ac ammeter in series with each electrolytic cap and quantify the average (ripple) current flowing in each cap. Then, I'd put a large (appropriate current rated) inductor in series with the input power and measure the same ripple currents again. A more meaningful test might be to measure the temperature of each electrolytic before and after the inductor was added. I'd be willing to bet each capacitor runs at least 30 degrees cooler with an inductor in series with the input power lead.

I wonder if newer ECU's use more appropriate methods of smoothing the input power such that reliability is not degraded by the use of large value electrolytic(s)?

Drop me a line, either direct or on this newsgroup.

TO repply directly send an e male to:

anonymoPusone aat thedoghousemail doot comP

Remove the upper case P's to get the proper email address to reply.

Regards.

Trabem

No, I do not work on ECU's as a profession. I'm a cert electronics tech, who professionnally works on mid range computers (25 + years experiance) down to component level. Cars are a hobby (I'm also cheap and don't trust dealers). I got into the ECU's when my kids cars started having problems, and the quick fix from the dealers was "replace the module, and give us lots of money". Even the auto wreckers back then were demanding a high price for ECU modules (if you could even find one for a Mitsu). Even Honda modules were difficult to get, (for a different reason, everyone was "upgrading"). One thing the computer industry taught me was how sensors and associated circuits worked, in disk drives, of 20 years ago. Virtually all the sensors found in a car today, were in disk drives and tape drives, 20 years ago (well except for the O2 sensor).

The problem with the caps is not the ripple or Mitsubishis fault. The capacitors have a design fault. This is an industry wide problem. I had to replace 13 caps on a 2001 Aptiva (IBM) computer motherboard because they were swelling up. I've seen this problem on numerous other circuits, (yes ripple will cause the same thing). My guess is that Mitsu just found a bad supplier of caps, for 5 odd years, by chance.

Check out this article :

this article; another; When I was working on my ECM, I thought the CPU voltage was 5.0 volts. There is an onboard requlator, in the form of a power transistor. The CPU doesn't operate on the 12 volt battery feed (well not directly) and the

5volt rail looked pretty clean on my scope. My problem was (twice, two different ECM's), there was a break in the power rail directly under the large cap, in the center of the PCB.

Lets not get over reactive here. I've been inside both Honda and Mazda ECU's. Generally they all work well, and ECU failures are rare , relative to the number produced, (unless somebody does something stupid on their car). Most ECU's have well protected input's and outputs, and the alternator ripple usually isn't a problem. High temp components are used because the temperature inside of a car, park in the sun, in say Arizona, will be well over 120 degrees F.

Electronics have been in cars for many decades (the audio system has been solid state since the sixties) so most of your concerns have been answered. If ripple was a problem (as it is in car audio) then they would have designed for it(as they did in the audio systems) in the ECU.

(potentiometer),